Cube heads

Yep.

![Ermagerd](http://puu.sh/dnD8g/936194a81c.png)

Of necessity I moved the resizing into the image manipulation functions, so that's now included in the benchmarks (hence the increase across the board). Getting the cube head is slower, but I thought it'd be much slower than it is; half the speed of getbody/bust isn't bad at all, considering how much more we're doing. So that's cool :)

```
BenchmarkSetup	2000000000	         0.00 ns/op
BenchmarkGetHead	  100000	     20421 ns/op
BenchmarkGetHelm	  100000	     21789 ns/op
BenchmarkGetBody	   50000	     49856 ns/op
BenchmarkGetBust	   50000	     57337 ns/op
BenchmarkGetCube	   10000	    117322 ns/op
```